#!/usr/bin/env python
# -*- coding: utf-8 -*-
# @Software: PyCharm
# @Version : Python-
# @Author  : Shengji He
# @Email   : hsjbit@163.com
# @File    : OddEvenLinkedList.py
# @Time    : 2020/11/13 10:06
# @Description:


# Definition for singly-linked list.
class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next


class Solution:
    def oddEvenList(self, head: ListNode) -> ListNode:
        """
        Given a singly linked list, group all odd nodes together followed by the even nodes. Please note here we 
        are talking about the node number and not the value in the nodes.
        
        You should try to do it in place. The program should run in O(1) space complexity and O(nodes) time complexity.
        
        Example 1:
        
            Input: 1->2->3->4->5->NULL
            Output: 1->3->5->2->4->NULL
        Example 2:
        
            Input: 2->1->3->5->6->4->7->NULL
            Output: 2->3->6->7->1->5->4->NULL

        Constraints:
            - The relative order inside both the even and odd groups should remain as it was in the input.
            - The first node is considered odd, the second node even and so on ...
            - The length of the linked list is between [0, 10^4].

        :param head: 
        :return: 
        """
        if not head:
            return head

        evenHead = head.next
        odd, even = head, evenHead
        while even and even.next:
            odd.next = even.next
            odd = odd.next
            even.next = odd.next
            even = even.next
        odd.next = evenHead
        return head


if __name__ == '__main__':
    S = Solution()
    head = ListNode(1)
    head.next = ListNode(2)
    head.next.next = ListNode(3)
    head.next.next.next = ListNode(4)
    head.next.next.next.next = ListNode(5)
    S.oddEvenList(head)
    print('done')
